Automobile type diesel engines employ high compression ratios such as 22:1. This high compression ratio is required only to start the engine and to keep the engine going at idle and light load operation. However, when the diesel engine is warm and at road or high load, the compression ratio could be reduced to approximately 12:1. The use of a low compression ratio would have the advantages of reducing friction and structural loading of the engine, and reducing NO.sub.x due to the lower temperature.
While it has been proposed to use a mixed cycle internal combustion engine operating on both an Otto and a diesel cycle, such proposals have not led to a successful commercial engine. The present invention is directed to an improved mixed cycle internal combustion engine which (1) reduces engine friction by reducing the compression ratio, (2) is insensitive to fuel quality and the engine has no octane, volatibility, or cetane requirement, (3) the engine starts easily and will idle without the diesel knock of conventional diesel engines, (4) thermal efficiency is about the same as conventional diesel engines but overall efficiency should be better than conventional diesels because of the reduced compression ratio and therefore reduced friction, (5) emission pollutants are low, (6) by using ignition means the combustion delay period is reduced and a better control of combustion is obtained, (7) the fuel is injected directly into the combustion chamber and preignition or knock is avoided thereby eliminating the octane number requirement, and (8) since the fuel is ignited by an ignition source the cetane number is no more of any significance.